The invention relates generally to filter units of the high efficiency particulate (HEPA) type capable of filtering submicron particulates, and more particularly to a thin HEPA filter unit made of sintered stainless steel metal that is washable in situ. The invention relates most preferredly to high pressure drop HEPA filters capable of being cleaned of particulates and restored to full flow operation in a short time, with a minimum of generated waste, and with a minimal number of parts and cleaning steps.
For clarity, references to HEPA filters herein are references to filters and/or filter systems capable of and routinely required to pass smoke testing demonstrating the removal from a fluid stream of about 99.97% of 0.3 micrometer (0.3 .mu.) dioctylphthalate (DOP) monodispersed particles. This is a standard definition familiar to those of skill in the art.
High Efficiency Particulate (HEPA) filters have been utilized for filtration of submicron particles from exhaust gas streams, and especially for the filtration of radiative particles from exhaust gases.
To date, makers and users of HEPA filters have focused on the development of filters that generate very low pressure drops in the course of removing particulate matter from the fluid stream. Low pressure drop has been thought to be necessary and advantageous because it requires only a relatively low power blower (or evacuator) system, and places little mechanical stress on the filter and on the filter support or framing structure. A consequence of requiring a low pressure drop, however, is a relatively low filtration rate measured, e.g., in cubic feet per minute. To compensate for these low flow-through rates, various configurations of filters and systems have been advanced to increase the overall flow rate by increasing the effective surface area of the filter. Thus, configurations such as accordion-folded, pleated, or other techniques have been suggested, each designed to enlarge the usable filter surface area while complying with space and other constraints imposed by the overall system.
Another problem faced by users of HEPA filters, and particularly users filtering exhaust gas streams from nuclear facilities, is filter life. Filter life is defined and determined with reference to a given filter's continued ability to provide sufficient fluid flow rates while continuing to prevent passage of the particulates to be filtered. For many industries and uses, the end of life for a HEPA filter constitutes a major expense and disposal problem. A filter in a nuclear facility, for example, is costly to replace in terms of time, manpower, and materials, including the temporary loss of use of the exhaust stream path. It also may pose a hazard due to the particulates entrapped or adhered to the filter, creating risks to personnel and a need for long term storage and disposal. Thus, extending the useful life of a HEPA filter presents a major saving in resources and safety to such facilities.
One approach to extending the life of a filter has been to provide means of cleaning or washing the filter, either in situ or upon removing the filter from its frame or mounting. The systems, methods, and apparatus known to date, however, each involve relatively complicated procedures or mechanisms, which are expensive and themselves liable to failures and breakdowns. The systems are subject to the parameters of the overall system, meaning that they must each work in a low pressure drop system. Other solutions include using modular components, allowing the filter to be taken off-line and cleaned. This too is highly expensive, and does not increase the expected life of each given filter, meaning that no real savings are achieved.
In Zeller, U.S. Pat. No. 5,487,771, a high efficiency metallic membrane of sintered nickel powder is disclosed. The membrane is sandwiched inside a frame, with improved porosity and gas throughput offered by the filter. In Layton, U.S. Pat. Nos. 5,238,477 and 5,158,586, HEPA filter units are disclosed having metallic membranes of continuous metal sheet accordion-folded to filter out submicron particles. The filters are resistant to elevated temperatures and are composed of stainless steel and other metals. In Davis, U.S. Pat. No. 5,114,447, an ultra-high efficiency particulate air filter is disclosed which is composed of multiple, porous, sintered metal filter discs manufactured from stainless steel, nickel and nickel alloys. The filter discs are enclosed permanently within a cylindrical casing, and the filter discs are resistant to high temperatures and pressures.
In Dillmann, et al., U.S. Pat. No. 4,865,803, a pressurized gas discharge filtration system is disclosed which includes stainless steel fiber filter packs aligned in multiple stages. The fiber filter packs are resistant to high temperatures. In Iniotakis, et al., U.S. Pat. No. 4,655,797, a microporous metallic membrane screen is disclosed for filtration of corrosive gas streams. The metallic membrane screens have metals such as gold or platinum catalytically deposited on the metal screens to provide corrosion resistance. In Komatsu, et al., U.S. Pat. No. 4,584,004, a chamber of particulate filters is disclosed, with the filters made of organic fiber material. A chamber is provided containing back-flushing nozzles for pulsed air cleaning of particulates entrapped inside the fiber filters.
In Weichselbaum, et al., U.S. Pat. No. 3,933,652, a process is disclosed for manufacturing a stainless steel filter for use in medical infusion equipment. The filter is made of sintered stainless steel particles, and is sealed inside a tubular fitting. In Spulgis, U.S. Pat. No. 3,881,899, an apparatus for dislodging particulates from a rechargeable filter is disclosed, where the filter is placed inside the apparatus, and the particles are removed from the filter using a pneumatic air delivery apparatus. The filter material in the particulate filter is finely divided activated charcoal.
In Seibert et al., U.S. Pat. No. 5,358,552, a process is disclosed for introducing a backwash liquid. The backwash liquid is forced through the filter in the direction opposite the normal flow of the stream to be filtered (in an upstream direction). The purpose is to flush the filter by forcing trapped particulates out of the filter in the direction from which the particulates became entrained in the filter media, thus cleaning the filter.
These and other prior filters have a variety of shortcomings. Some of them, while capable of relatively full cleaning, are incapable of achieving HEPA standards and/or of withstanding the required operating environment. Others are incapable of being cleaned, or at least cannot be cleaned to even a relatively high percentage of full flow and filtering ability. Others must simply be replaced, while many cannot be cleaned in situ. Of those that can be cleaned in situ, they require very complicated backflushing mechanism, requiring not only the installation of additional components, but requiring the installation of complicated recovery systems.
There is thus a need in the art for a HEPA filter that is capable of providing the required filtration and is resistant to damage, that can be cleaned repeatedly in situ at low cost and low complexity, that can be cleaned and restored to full filtering capability at the pressure drop required, and that will allow recovery of the removed particulates for reclamation and/or analysis.